The present invention relates generally to a display apparatus using electroluminescence elements. More particularly, the present invention relates to a display apparatus which displays high-definition images.
Electroluminescence (EL) elements include an inorganic EL element which uses a thin film of an inorganic compound like ZnS or ZnSe as a light emitting material, and an organic EL element which uses an organic compound as a light emitting material. The organic EL element has the following features: (1) high light emitting efficiency, (2) low driving voltage, (3) ability to display multicolors (green, red, blue, yellow, etc.) by selection of a light emitting material in use, (4) clear display and no need for a back light because it is of a self light emission type, (5) planar light emission and no dependency on the angle of visibility, (6) Physically thin and light, and (7) decreasing temperature in the fabrication process, which allows a soft material like a plastic film to be used for the substrate. A display apparatus using an organic EL element having the aforementioned properties (hereinafter called an "organic EL display apparatus") has recently been getting attention as a replacement for a CRT or LCD display device.
An organic EL display apparatus can employ either a simple matrix system or an active matrix system. The simple matrix system allows an external driving unit to directly drive organic EL elements as a matrix of pixels, arranged on a display panel, in synchronism with a scan signal. Because the display panel of any display apparatus which employs such a system includes organic EL elements, the driving time (duty) assigned to each pixel decreases as the number of scan lines is increased, which lowers the contrast of an image displayed on the display panel.
The active matrix system has the elements (active elements) for driving pixels, respectively, for a matrix of pixels. The element serves as a switch which is switched on or off by the scan signal. When an element for driving a pixel is enabled, a data signal (display signal, video signal) is transmitted and written, via the element for driving pixel, to an anode of the associated organic EL element. The organic EL element is driven in this manner. When the element for driving pixel is disabled later, the data signal applied to the anode of the organic EL element is held as a charge in the organic EL element. The organic EL element is kept driven by the discharging of charges until the associated element for driving pixel is switched on again. Thus, even though the driving time per pixel becomes shorter when the number of scan lines is increased, the driving of the organic EL elements is not affected. As a result, the contrast is prevented from becoming lower. In this respect, the active matrix system displays images with a higher quality than the simple matrix system.
The active matrix system employs transistor type (three-terminal type) elements for driving the pixels or diode type (two-terminal type) elements for driving the pixels. The transistor type is characterized by easy acquisition of high contrast and high resolution, but difficult to fabricate, as compared with the diode type. That is, the transistor type organic EL display apparatus provides high-definition images which match with those displayed by a CRT. The operational principle of the active matrix system is mainly associated with the transistor type elements for driving pixels.
FIG. 1 is a schematic cross-sectional perspective view showing a part of a prior art organic EL display apparatus 101 according to a simple matrix system. FIG. 2 is a schematic cross-sectional view taken along line 2--2 in FIG. 1.
Referring to FIGS. 1 and 2, a plurality of anodes 103 formed of transparent electrodes of ITO (Indium Tin Oxide) or the like and a first hole transporting layer 104 of MTDATA (4,4'-bis(3-methylphenylphenylamino)biphenyl) are provided on a transparent insulator substrate 102 of glass, synthetic resin or the like. A second hole transporting layer 105 of TPD (4,4',4"-tris(3-methylphenylphenylamino) triphenylamine) is located on the first hole transporting layer 104. Located on the second hole transporting layer 105 is a light emitting layer 106 of Bebq2(10-benzo[h]quinolinol-beryllium complex) containing a quinacridone dielectric substance. Provided on the light emitting layer 106 is an electron transporting layer 107 of Bebq2. A plurality of cathodes 108 of a magnesium-indium alloy are located on the electron transporting layer 107. The layers 104 to 107 each of an organic compound, the anodes 103, and the cathodes 108 form an organic EL element 109.
The individual anodes 103 are arranged parallel to one another. The individual cathodes 108 are likewise arranged parallel to one another, and perpendicular to the anodes 103.
As the holes from the anodes 103 are recombined with the electrons from the cathodes 108 inside the light emitting layer 106, the light emitting layer 106 generates light. The light from the light emitting layer 106 is emitted outside from the anodes 103 through the transparent insulator substrate 102 as indicated by the arrow .tau. (FIG. 2).
The first and second hole transporting layers 104 and 105 facilitate the injection of holes into the light emitting layer 106 from the anodes 103. The electron transporting layer 107 facilitates the injection of electrons into the light emitting layer 106 from the cathodes 108. An organic EL display apparatus 101 having the structure permits green light to be emitted at higher external quantum efficiency and improved intensity.
FIG. 3 is a plan view of the organic EL display apparatus 101, as viewed from the anodes 103. In FIG. 3, only the anodes 103 (103a-103c) and the cathodes 108 (108a-108c) are illustrated. The individual intersections of anodes 103a to 103c and cathodes 108a to 108c are defined as light emitting areas B or pixels. In the simple matrix system, the positive terminal of a driving power supply (not shown) is connected to the anodes 103 corresponding to the light emitting areas B, while the negative terminal of the driving power supply is connected to the corresponding cathodes 108. As the anodes 103 and cathodes 108 are energized, the desired pixels emit light.
For example, if the positive terminal of the driving power supply is connected to the anode 103b and the power-supply's negative terminal is connected to the cathode 108a, power is provided to the anode 103b and the cathode 108a, and a forward current flows, as indicated by the arrow .alpha., causing the light emitting area B at the intersection C to emit light. At this time, a leak current may flow as indicated by the arrow .beta., energizing the light emitting area B at an intersection close to the intersection C. As a result, the light emitting areas B at the intersection D between the anode 103a and cathode 108a, the intersection E between the anode 103c and cathode 108a, and at the intersection F between the anode 103b and cathode 108b, as well as the light emitting area B at the intersection C may emit light. This phenomenon is called optical crosstalk caused by the leak current characteristic of the EL element.
Further, light emitted from the light emitting layer 106 may be scattered inside the electron transporting layer 107 and reflected at some cathode 108 to go outside, as indicated by the arrow .delta. (FIG. 2). Furthermore, light emitted from the light emitting layer 106 may be scattered inside the first and second hole transporting layers 104 and 105, and go out, as indicated by the arrow .epsilon. (FIG. 2). In addition, light emitted from the light emitting layer 106 may optically be guided out by the optical waveguide effect which originates from the difference between the refractive indexes of the light emitting layer 106 and the first and second hole transporting layers 104 and 105, as indicated by the arrow .eta.. Such radiative actions bring about undesirable light emission from a non-radiative area. This phenomenon is called optical crosstalk caused by the light scattering originated from the structure of the EL element.
The aforementioned two optical crosstalks deteriorate the contrast of the organic EL display apparatus 101, thereby lowering the resolution. This makes it difficult to acquire high-definition images. In a full-color display apparatus, particularly, color "bleeding" occurs, which disables the acquisition of clear images. Such a shortcoming occurs not only in the simple matrix system, but also in the active matrix system, and may occur in an inorganic EL display apparatus as well as an organic EL display apparatus.
Broadly speaking, the present invention relates to a display apparatus using electroluminescence elements, which displays high-definition images.